New model calculations for the production rates of cosmogenic nuclides in iron meteorites

Abstract— Here we present the first purely physical model for cosmogenic production rates in iron meteorites with radii from 5 cm to 120 cm and for the outermost 1.3 m of an object having a radius of 10 m. The calculations are based on our current best knowledge of the particle spectra and the cross sections for the relevant nuclear reactions. The model usually describes the production rates for cosmogenic radionuclides within their uncertainties; exceptions are ⁵³Mn and ⁶⁰Fe, possibly due to normalization problems. When an average S content of about 1 ± 0.5% is assumed for Grant and Carbo samples, which is consistent with our earlier study, the model predictions for ³He, ²¹Ne, and ³⁸Ar are in agreement. For ⁴He the model has to be adjusted by 24%, possibly a result of our rather crude approximation for the primary galactic α particles. For reasons not yet understood the modeled ³⁶Ar/³⁸Ar ratio is about 30–40% higher than the ratio typically measured in iron meteorites. Currently, the only reasonable explanation for this discrepancy is the lack of experimentally determined neutron induced cross sections and therefore the uncertainties of the model itself. However, the new model predictions, though not yet perfect, enable determining the radius of the meteoroid, the exposure age, the sulphur content of the studied sample as well as the terrestrial residence time. The determination of exposure ages is of special interest because of the still open question whether the GCR was constant over long time scales. Therefore we will discuss in detail the differences between exposure ages determined with different cosmogenic nuclides. With the new model we can calculate exposure ages that are based on the production rates (cm³STP/(gMa)) of noble gases only. These exposure ages, referred to as noble gas exposure ages or simply ^3,4He, ²¹Ne, or ^36,38Ar ages, are calculated assuming the current GCR flux. Besides calculating noble gas ages we were also able to improve the ⁴¹K‐⁴⁰K‐and the ³⁶Cl‐³⁶Ar dating methods with the new model. Note that we distinguish between ³⁶Ar ages (calculated via ³⁶Ar production rates only) and ³⁶Cl‐³⁶Ar ages. Exposure ages for Grant and Carbo, calculated with the revised ⁴¹K‐⁴⁰K method, are 628 ± 30 Ma and 841 ± 19 Ma, respectively. For Grant this is equal to the ages obtained using ³He, ²¹Ne, and ³⁸Ar but higher than the 3⁶Ar‐ and ³⁶Cl‐³⁶Ar ages by ?30%. For Carbo the ⁴¹K‐⁴⁰K age is ?40% lower than the ages obtained using ³He, ²¹Ne, and ³⁸Ar but equal to the ³⁶Ar age. These differences can either be explained by our still insufficient knowledge of the neutron‐induced cross sections or by a long‐term variation of the GCR.     

Noble gases in Grant and Carbo and the influence of S‐ and P‐rich mineral inclusions on the 41K‐40K dating system

Abstract— Cosmogenic He, Ne, and Ar were measured in the iron meteorites Grant (IIIAB) and Carbo (IID) to re‐determine their preatmospheric geometries and exposure histories. We also investigated the influence of sulphur‐ and/or phosphorus‐rich inclusions on the production rates of cosmogenic Ne. Depth profiles measured in Grant indicate a preatmospheric center location 117 mm left from the reference line and 9 mm below bar B, which is clearly different (?10 cm) from earlier results (?165 mm left from the reference line on bar F). For Carbo the preatmospheric center location was found to be 120 mm right of the reference line and 15 mm above bar J, which is in agreement with literature data. The new measurements indicate a spherical preatmospheric shape for both meteorites and, based on literature ³⁶C1 data, the radii were estimated to be about 32 cm and 70 cm for Grant and Carbo, respectively. We demonstrate that minor elements like S and P have a significant influence on the production rates of cosmogenic Ne. In our samples, containing on average 0.5% S and/or P, about 20% of ²¹Ne was produced from these minor elements. Using measured ²¹Ne concentrations and endmember ²²Ne/²¹Ne ratios for Fe + Ni and S + P, respectively, we show that it is possible to correct for ²¹Ne produced from S and/or P. The thus corrected data are then used to calculate new ⁴¹K‐⁴⁰K exposure ages—using published K data—which results in 564 ± 78 Ma for Grant and 725 ± 100 Ma for Carbo. The correction always lowers the ²¹Ne concentrations and consequently decreases the ⁴¹K‐⁴⁰K exposure ages. The discrepancies between ³⁶Cl‐³⁶Ar and ⁴¹K‐⁴⁰K ages are accordingly reduced. The existence of a significant long‐term variation of the GCR, which is based on a former 30–50% difference between ⁴¹K‐⁴⁰K and ³⁶Cl‐³⁶Ar ages, may warrant re‐investigation.     

Sorption of Np(V) and Np(IV) onto kaolinite: Effects of pH,ionic strength,carbonate and humic acid

The sorption of Np(V) and Np(IV) onto kaolinite has been studied in the absence and presence of humic acid (HA) in a series of batch equilibrium experiments under different experimental conditions: [Np]₀: 1.0 × 10^-6 or 1.0 × 10^-5 M, [HA]₀: 0 or 50 mg/L, I: 0.01 or 0.1 M NaClO₄, solid to liquid ratio: 4 g/L, pH: 6–11, anaerobic or aerobic conditions, without or with carbonate. The results showed that the Np(V) sorption onto kaolinite is affected by solution pH, ionic strength, Np concentration, presence of carbonate and HA. In the absence of carbonate, the Np(V) uptake increased with pH up to ∼96% at pH 11. HA further increased the Np(V) sorption between pH 6 and 9 but decreased the Np(V) sorption between pH 9 and 11. In the presence of carbonate, the Np(V) sorption increased with pH and reached a maximum of 54% between pH 8.5 and 9. At higher pH values, the Np(V) sorption decreased due to the presence of dissolved neptunyl carbonate species with a higher negative charge that were not sorbed onto the kaolinite surface which is negatively charged in this pH range. HA again decreased the Np(V) uptake in the near-neutral to alkaline pH range due to formation of aqueous neptunyl humate complexes. The decrease of the initial Np(V) concentration from 1.0 × 10⁻⁵ M to 1.0 × 10⁻⁶ M led to a shift of the Np(V) adsorption edge to lower pH values. A higher ionic strength increased the Np(V) uptake onto kaolinite in the presence of carbonate but had no effect on Np(V) uptake in the absence of carbonate.     

Geochemistry and Sr, Nd, Pb isotopic composition of the Central Atlantic Magmatic Province (CAMP) in Guyana and Guinea

The Central Atlantic Magmatic Province (CAMP) is one of the largest igneous provinces on Earth, extending more than 5000 km north to south, on both sides of the Atlantic Ocean. Its emplacement occurred about 200 Ma ago, at the Triassic–Jurassic boundary, and is linked to the initial breakup of Pangaea. Two areas of the province are studied here: French Guyana/Surinam (South America) and Guinea (West Africa), in order to document the petrogenesis and geodynamical significance of high-Ti and low-Ti basaltic magmas from the CAMP.

In Guyana, doleritic and gabbroic dykes are located on the edge of the Guiana Shield, and represent limited volumes of magma. They display low SiO₂ (47–50%), high TiO₂ (2.5–3.5%) and high FeO tholeiitic trends and show variably enriched trace element patterns ((La/Yb)_n=1.5–5.1). Their isotopic signature and ratios of very incompatible elements (εNd_i=+5.8 to +4.2, (⁸⁷Sr/⁸⁶Sr)_i=0.703–0.705, (²⁰⁷Pb/²⁰⁴Pb)_i=15.46–15.64) match a depleted PREMA (prevalent mantle)-like source. Their genesis can be modeled by ca. 15% partial melting of a lherzolite source, and a subsequent limited fractional crystallization (5–10%) or a slight upper crustal assimilation–fractional crystallization (AFC, r=0.1, Proterozoic contaminant). In Guinea, in contrast, huge volumes of CAMP magmas were intruded along the Rockelides suture and the West African craton, forming the Fouta Djalon sills and the Kakoulima laccolith. The laccolith is more than 1000 m thick. These features consist of gabbros, dolerites, diorites and mafic (gabbro) and ultramafic (dunite, wherlite) cumulates. Guinean tholeiites show high SiO₂ (51–58%), low TiO₂ (0.7–1.2%) and FeO trends, with high LILE/HFSE ratios and slight negative Nb–Ta anomalies. Isotopic signatures (εNd_i=+0.4 to −5.3, (⁸⁷Sr/⁸⁶Sr)_i=0.705–0.710, (²⁰⁷Pb/²⁰⁴Pb)_i=15.57–15.66) indicate a more enriched source than for Guyana as well as a higher rate of magma–upper crust interaction through an AFC process (r=0.3, Birimian crust contaminant) and, probably, an additional upper crustal contamination for the most differentiated sample.

This geochemical study supports the prevalence in Guinea, as for other low-Ti CAMP tholeiites, of a lithospheric mantle source, previously enriched during ancient subduction events, and preferentially reactivated in late Triassic times by edge-driven convection between cratonic and mobile belt domains. A larger contribution from a depleted asthenospheric source is required to generate high-Ti tholeiites in Guyana, which may reflect the development of CAMP rifting towards the initiation of the Central Atlantic oceanic crust.     

Eemian crustal deformation in the eastern Baltic area in the light of the new sites at Peski, Russia and Põhja-Uhtju, Estonia

Several Eemian (Mikulino) marine deposits are known from the northwestern part of Russia and from Estonia. The best-known deposits are situated at Mga, Russia and at Prangli, Estonia. Two new sites with clayey and silty deposits covered by till were studied for pollen and diatoms at Peski, Russia and Põhja-Uhtju, Estonia. At Peski, the deposit representing the Eemian Interglacial is 3.8 m thick at the depth of 13.4–9.6 m above present sea-level. At Põhja-Uhtju, the deposit representing the Eemian is 3.5 m thick at the depth of 47.9–51.4 m below present sea-level. Although Peski is situated at a higher altitude than Põhja-Uhtju at present, the diatom stratigraphy at these sites indicates deeper and more saline conditions in the Peski area than at Põhja-Uhtju during the Eemian. This result is similar to some previous studies, which indicate, that although the Russian deposits (e.g. Peski, Mga) are now at a higher altitude than those in Estonia (Põhja-Uhtju and Prangli), the diatoms in the Russian deposits are indicative of a considerable depth of water during the time of deposition. These deposits suggest that the Eemian shore levels ascend from Estonia eastwards, while the Late Weichselian and Holocene shorelines tilt downwards in the same general direction. The present material from Estonia and northwestern part of Russia shows marked differences between the Eemian and Late Weichselian/Holocene crustal deformations, which probably resulted from different ice loads during the final glaciation phases and probably also from different deglaciation patterns during the Saalian and Late Weichselian.     

Extremes of near-surface wind speed over Europe and their future changes as estimated from an ensemble of RCM simulations

In this study, we analyse the uncertainty of the effect of enhanced greenhouse gas conditions on windiness projected by an ensemble of regional model simulations driven by the same global control and climate change simulations. These global conditions, representative for 1961–1990 and 2071–2100, were prepared by the Hadley Centre based on the IPCC SRES/A2 scenario. The basic data sets consist of simulated daily maximum and daily mean wind speed fields (over land) from the PRUDENCE data archive at the Danish Meteorological Institute. The main focus is on the results from the standard 50 km-resolution runs of eight regional models. The best parameter for determining possible future changes in extreme wind speeds and possible change in the number of storm events is maximum daily wind speed. It turned out during this study that the method for calculating maximum daily wind speed differs among the regional models. A comparison of simulated winds with observations for the control period shows that models without gust parameterisation are not able to realistically capture high wind speeds. The two models with gust parametrization estimate an increase of up to 20% of the number of storm peak (defined as gusts?≥?8 Bft in this paper) events over Central Europe in the future. In order to use a larger ensemble of models than just the two with gust parameterisation, we also look at the 99th percentile of daily mean wind speed. We divide Europe into eight sub-regions (e.g., British Isles, Iberian Peninsula, NE Europe) and investigate the inter-monthly variation of wind over these regions as well as differences between today’s climate and a possible future climate. Results show differences and similarities between the sub-regions in magnitude, spread, and seasonal tendencies. The model ensemble indicates a possible increase in future mean daily wind speed during winter months, and a decrease during autumn in areas influenced by North Atlantic extra-tropical cyclones.     

Raman study of radiation-damaged zircon under hydrostatic compression

Pressure-induced changes of Raman band parameters of four natural, gem-quality zircon samples with different degrees of self-irradiation damage, and synthetic ZrSiO₄ without radiation damage, have been studied under hydrostatic compression in a diamond anvil cell up to ~10 GPa. Radiation-damaged zircon shows similar up-shifts of internal SiO₄ stretching modes at elevated pressures as non-damaged ZrSiO₄. Only minor changes of band-widths were observed in all cases. This makes it possible to estimate the degree of radiation damage from the width of the ν₃(SiO₄) band of zircon inclusions in situ, almost independent from potential “fossilized pressures” or compressive strain acting on the inclusions. An application is the non-destructive analysis of gemstones such as corundum or spinel: broadened Raman bands are a reliable indicator of self-irradiation damage in zircon inclusions, whose presence allows one to exclude artificial color enhancement by high-temperature treatment of the specimen.     

Future productivity and phenology changes in European grasslands for different warming levels: implications for grassland management and carbon balance

Background

Europe has warmed more than the global average (land and ocean) since pre-industrial times, and is also projected to continue to warm faster than the global average in the twenty-first century. According to the climate models ensemble projections for various climate scenarios, annual mean temperature of Europe for 2071–2100 is predicted to be 1–5.5 °C higher than that for 1971–2000. Climate change and elevated CO₂ concentration are anticipated to affect grassland management and livestock production in Europe. However, there has been little work done to quantify the European-wide response of grassland to future climate change. Here we applied ORCHIDEE-GM v2.2, a grid-based model for managed grassland, over European grassland to estimate the impacts of future global change.

Results

Increases in grassland productivity are simulated in response to future global change, which are mainly attributed to the simulated fertilization effect of rising CO₂. The results show significant phenology shifts, in particular an earlier winter-spring onset of grass growth over Europe. A longer growing season is projected over southern and southeastern Europe. In other regions, summer drought causes an earlier end to the growing season, overall reducing growing season length. Future global change allows an increase of management intensity with higher than current potential annual grass forage yield, grazing capacity and livestock density, and a shift in seasonal grazing capacity. We found a continual grassland soil carbon sink in Mediterranean, Alpine, North eastern, South eastern and Eastern regions under specific warming level (SWL) of 1.5 and 2 °C relative to pre-industrial climate. However, this carbon sink is found to saturate, and gradually turn to a carbon source at warming level reaching 3.5 °C.

Conclusions

This study provides a European-wide assessment of the future changes in productivity and phenology of grassland, and their consequences for the management intensity and the carbon balance. The simulated productivity increase in response to future global change enables an intensification of grassland management over Europe. However, the simulated increase in the interannual variability of grassland productivity over some regions may reduce the farmers’ ability to take advantage of the increased long-term mean productivity in the face of more frequent, and more severe drops of productivity in the future.

     

Nondestructive imaging of hypervelocity impact-induced damage zones beneath laboratory-created craters by means of ultrasound travel-time tomography

Since the 1960s, hypervelocity impact experiments have been conducted to study the complex deformation mechanisms which occur in the subsurface of meteorite craters. Here, we present ultrasound tomography measurements of the damage zone underneath seven experimentally produced impact craters in sandstone cubes. Within the framework of the Multidisciplinary Experimental and Modeling Impact Research Network and the NEOShield Project, decimeter-sized sandstone targets were impacted by aluminum and steel projectiles with radii of 2.5, 4, and 5 mm at velocities between ~3.0 and ~7.4 km s⁻¹. The 2-D ultrasound tomography clearly shows a correlation between impact energy and the damaged volume within the target blocks. When increasing impact energies from 805 to 2402 J, a corresponding increase in the damage radius from ~13.1 cm to ~17.6 cm was calculated. p-Wave velocity reductions up to 18.3% (for the highest impact energy) were observed in the vicinity of the craters. The reduction in seismic velocity decreased uniformly and linearly with increasing distance from the impact point. The damage intensities correspond to peak damage parameters of 0.4–0.51 compared to undamaged target blocks. In addition to the damage zone below the crater, we could identify weakened zones at the sandstone walls which represent precursors of spalling. The volume of the damaged subsurface beneath experimentally produced craters determined through ultrasound tomography is larger than that obtained from previously reported p-wave velocity reductions or to microscopic and microcomputed tomography observations of crack densities in experimentally produced craters.     

Effects of nanoparticles in Mytilus edulis gills and hepatopancreas - a new threat to marine life?

Every day new extraordinary properties of nanoparticles (a billionth of a meter) are discovered and worldwide millions are invested into nanotechnology and nanomaterials. Risks to marine organisms are still not fully understood and biomarkers to detect health effects are not implemented, yet. We used the filter feeding blue mussel as a model to analyse uptake and effects of nanoparticles from glass wool, a new absorbent material suggested for use in floating oil spill barriers. In both, gills and hepatopancreas we analysed uptake of nanomaterials by transmission electronmicroscopy (TEM) in unstained ultrathin sections over a period of up to 16 days. Lysosomal stability and lipofuscin content as general indicators of cellular pathology and oxidative stress were also measured. As portals of uptake, diffusion and endocytosis were identified resulting in nanoparticle accumulation in endocytotic vesicles, lysosomes, mitochondria and nuclei. Dramatic decrease of lysosomal membrane stability occurred after 12h of exposure. Lysosomal damage was followed by excessive lipofuscin accumulation indicative of severe oxidative stress. Increased phagocytosis by granulocytes, autophagy and finally apoptosis of epithelial cells of gills and primary and secondary digestive tubules epithelial cells indicated progressive cell death. These pathological responses are regarded as general indices of toxic cell injury and oxidative stress. By the combinational use of biomakers with the ultrastructural localisation of nanoparticle deposition, final evidence of cause-effect relationships is delivered.